1. Field of the Invention
This invention relates to a fuel supply system for use in an automotive vehicle equipped with an internal combustion engine and, more particularly, to such a fuel supply system responsive to various vehicle running conditions for shutting off the flow of fuel for the engine.
2. Description of the Prior Art
Fuel supply systems such as fuel injection system or carburetor incorporated in automotive vehicle internal combustion engine have been provided which have fuel cut-off control apparatus responsive to various vehicle running conditions for shutting off the flow of fuel for the engine during deceleration for higher fuel economy and exhaust gas purifying performance.
Such conventional fuel cut-off control apparatus has inputs from an engine speed sensor and an engine temperature sensor for comparing the actual engine speed with high and low reference levels varying with engine temperature. The conventional fuel cut-off control apparatus also have an additional input from an idle switch adapted to become conductive when the throttle valve is in its fully closed position or the accelerator pedal is released. During deceleration, the fuel cut-off control apparatus cuts off the flow of fuel from the fuel supply system to the engine if the idle switch is turned on at an engine speed above the high reference level and releases the fuel-cut condition when the engine speed falls below the low reference level.
One difficulty with such conventional apparatus is their tendency toward hunting which results in repeated interruption and admission of the fuel flow to the engine in a short time especially when the engine is warming-up or running with its transmission placed in a low or second gear position. This causes unsmooth vehicle running and has an adverse effect upon engine durability and exhaust gas purifying performance.
This is due to the hunting provision of a fast-idle machanism associated with the engine for increasing the engine speed to ensure stable engine rotation during engine warming-up conditions. That is, the fast-idle mechanism increases the engine speed above the high reference level to allow the apparatus to shut off the fuel flow for the engine. As a result, the engine speed falls below the low reference level to cause the apparatus to admit the fuel flow to the engine. These conditions are repeated in a relatively short time at idle conditions. In order to avoid the above disadvantages, it is required to produce a fast-idle mechanism with extremely high accuracy, which results in a complex and expensive mechanism.
Furthermore, the engine may be operated, with its condition repeatedly changed between acceleration and deceleration, while the automotive vehicle is running with the transmission in low gear due to heavy traffic. The engine speed readily exceeds the high reference level with the accelerator pedal depressed and falls below the high reference level with the accelerator pedal released, causing the apparatus to cut off the fuel flow to the engine. Then, the engine speed immediately falls below the low reference level due to strong engine braking to cause the apparatus to allow the fuel flow to the engine. This spoils smooth vehicle running.